A room-temperature EPR (electron paramagnetic resonance) study on the charring (under He flow) and char transformation (by air) of pectin, cellulose, and tobacco is reported. The EPR intensities of pectin and tobacco chars (with charring temperatures from 250 degreesC to 600 degreesC) first increase rapidly and then decrease slowly during the room-temperature air exposure, indicating that the air exposure of pectin and tobacco chars involves two different types of chemical processes: the production and annihilation of free radicals. During air exposure, "new" radicals form at nonradical sites of the char surface, not through direct reactions Of O-2 with "old" free radicals generated by pyrolysis. These new free radicals have larger g-values than the old radicals, implying an oxygen-centered structure for the former. The low-temperature chars of pure cellulose show only an apparently small, slow decrease in radical concentration upon air exposure. For the high-temperature (> 400 degreesC) chars of cellulose, air exposure is accompanied by a large reduction in apparent radical concentration. Possible free radical structures and processes are considered.